1. Field of the Invention
The present invention relates to a resist composition, more particularly, it relates to a high energy radiation-sensitive, pattern-forming resist composition. This resist composition is particularly useful as a top layer resist in a bi-level resist system because, after a pattering exposure, the resist can be stably developed due to a remarkably increased difference of the solubility in the developer of the exposed and unexposed areas thereof. The present resist composition has an excellent resistance to oxygen reactive ion etching (O.sub.2 RIE) The present invention also relates to a process for the formation of resist patterns using the resist composition of the present invention. The resist composition and pattern formation process of the present invention can be advantageously utilized in particular in the process of the formation of multilayer wiring during the production of semiconductor devices such as integrated circuits (ICs), large-scale integrated circuits (LSIs), and ultra-large-scale integrated circuits (ULSIs).
2. Description of the Related Art
A thin-film formation technology and photolithography or electron beam lithography are widely utilized in the production of electronic circuit devices with fine circuit patterns, for example, semiconductor devices, magnetic bubble memory devices and surface wave filter devices, and as an example thereof, the present description is of a production of wiring patterns. Namely, an electrically conductive layer, electrically insulating layer or other thin layer(s) are formed on a substrate by conventional physical methods such as vacuum deposition or sputtering, or conventional chemical methods such as chemical vapor deposition (CVD), and after the formation of such wiring layer, a resist is spin-coated to form a resist layer. The resist layer is then pattern-wise exposed to radiations such as ultraviolet (UV) radiations, and then developed to form resist patterns. Subsequent to the formation of resist patterns, the underlying wiring layer is wet-etched or dry-etched, using the resist patterns as a mask, and thus conductive fine patterns, insulating fine patterns or other fine patterns are formed on the substrate. Note, the term "substrate" used herein is intended to mean the substrate itself or the electrically conductive layer, electrically insulating layer or other thin layer(s) formed on the substrate.
Recently, to satisfy the requirements for a minimum pattern width of less than 1 .mu.m in the resist patterns, an electron beam exposure is frequently used instead of a UV exposure to form resist patterns. Namely, after coating of an electron beam-sensitive resist, an electron beam having a reduced beam diameter is scanned over the resist to conduct a direct pattern-wise exposure. The EB-exposed resist is then developed with a suitable developer, and superior fine resist patterns are thus obtained. Note, a wavelength of the electron beam can vary depending upon the specific level of the applied voltage, but generally is about 0.1 .ANG..
Referring to the above-described formation of wiring patterns, this is generally carried out on a single-layer resist system, but this system is not suitable for the production of recently developed semiconductor devices because, in the process for the production of recent semiconductor devices such as very-large-scale integrated circuits (VLSIs) or ULSIs, to increase a degree of integration of the circuits, conventionally the wiring is constituted as multilayer wiring, and as a result, step portions having a height of 1 to 2 .mu.m are formed on a surface of the substrate. This formation of the stepped portions is a bar to the obtaining of fine resist patterns with a high accuracy. Note, the single-layer resist system can not remove this bar.
To solve the problem due to the stepped portions on the substrate surface, a bi-level resist system was developed, and is now widely utilized in the production of VLSIs, ULSIs and other devices. Generally, the bi-level resist system is carried out as follows.
First, an organic resin which can be easily dry etched with oxygen (O.sub.2) plasma, such as a phenolic novolak resin, is spin-coated at a layer thickness of about 2 .mu.m on a substrate such as a semiconductor substrate, and baked to form a bottom layer resist, whereby an uneven surface of the substrate is levelled. Thereafter, a top layer resist is coated at a thickness of about 0.2 to 0.3 .mu.m over the surface of the bottom layer resist to form a bi-level resist structure. Various polymeric materials can be used as the top layer resist. The positive-working EB resist used as the top layer resist can be dissolved in a developer as a result of a session of polymer chain upon the EB exposure, but the unexposed area of the top layer resist can retain the excellent resistance thereof to O.sub.2 plasma, since the resist in this area is not exposed to EB.
After the formation of the bi-level resist structure, the top layer resist is pattern-wise exposed and developed to form a pattern of the top layer resist, and then, using this pattern as a mask, the underlying bottom layer resist is selectively etched with O.sub.2 plasma. The pattern of the upper layer resist is transferred to the bottom layer resist.
In this bi-level resist system, since the bottom layer resist can prevent an undesirable influence of the steps of the underlying substrate on the patterning, and a reflection of exposing radiations by the substrate surface, and further, the top layer resist can be used at a reduced layer thickness, it becomes possible to remarkably increase the resolution of the resist in comparison with the single-layer resist system.
As a previously described, various polymers can be used as the top layer resist. Nevertheless, none thereof fully satisfies the requirements for the top layer resist to be exposed to high energy radiations such as electron beams and X-rays. For example, Tanaka et al teach in Preprint, 29p-H-6, of 1985 Spring Symposium of Japan Society of Applied Physics, that the polymer of the following formula (IV), i.e., a polymer of .alpha.-substituted acrylate containing a silicon atom in an ester moiety thereof, can be used as a positive-working resist having an excellent resistance to O.sub.2 RIE (oxygen reactive ion etching): ##STR2## in which
R' represents a hydrocarbon group exclusive of hydrogen, a halogen atom or halogenated alkyl group,
R" represents a silicon-containing alkyl group such as --CH.sub.2 Si(CH.sub.3).sub.3, and n is an integer. It was found, however, that many of the resist polymers of the formula (IV) can be rapidly dissolved in an organic solvent (within several seconds), and therefore, they can not be stably developed after an exposure thereof.
To avoid this problem, Tanaka et al also teach in the same Preprint 29p-H-6 that methacrylic acid of the formula: ##STR3## can be mixed with .alpha.-substituted acrylate of the formula: ##STR4## to form the polymer of the following formula (V): ##STR5## in which m and n each is an interger. After coating and before exposure, the resist polymer is heated or baked at an elevated temperature to cause a crosslinking thereof. Since the crosslinking can modify a solubility of the coated resist polymer in a developer, it become possible to conduct a more stable development than with the resist polymers of the formula (IV), i.e., to increase the types of solvents used as the developer. Nevertheless, the resist polymers of the formula (V) suffer from serious problems. Namely, since they have a low sensitivity and resolution, the resist polymers can not be used to exactly reproduce fine patterns. It is therefore desired to provide an improved resist polymer having a high sensitivity, resolution and resistance to O.sub.2 RIE, and enabling a stable development.